The direct measurement of oxygen uptake and respiratory quotient is often of importance in medical research or medical care. Many methods are available to make this measurement, but most of them suffer from serious defects. To make the measurement directly from the airway, it is necessary to know the respiratory minute volume, and also the exact concentration of mean inspired gas and mean expired gas for oxygen and carbon dioxide. The measurement of this mean volume and of the two gas concentrations usually requires bulky spirometers and other large apparatus, or else depends upon high flow past the nose and mouth, with downstream sampling. Alternately, the gas concentrations can be measured continuously during inspiration and expiration with the measured concentrations multiplied by the amount of flow and then integrated to obtain the necessary values; but this requires a gas analyzer of extremely rapid response and very careful control of all flows in the system. Because of the careful control required such systems have not proved very accurate.